According to a conventional tandem type color printer, four photosensitive drums for the colors of yellow magenta, cyan and black can be integrally attached to or detached from a main casing.
In such type of color printer, a frame can be pulled out in a horizontal direction from the main casing, and four photosensitive drums are held in the frame and arrayed in a pull-out direction of the frame. Further, developing cartridges provided with developing rollers and corresponding to these photosensitive drums are detachably mounted on the frame from the above. If toner in a developing cartridge gets empty, the developing cartridge is taken out of the frame, and a new developing cartridge is mounted on the frame.